How to investigate Microsoft Outlook & Exchange Online evidence
Microsoft delivery failures can involve message authentication, recipient state, tenant policy, reputation or routing. The most useful workflow combines the exact NDR with sender-domain evidence instead of applying one explanation to every 5.x.x code.
Signals worth checking
- NDR status and diagnostic text
- SPF, DKIM and DMARC state
- Visible From and authenticated identity
- MX and sender-service dependencies
- Legacy SMTP and migration dependencies
Diagnostic path
- Classify the exact NDR before changing DNS.
- Separate recipient or tenant errors from sender-authentication errors.
- Inspect a delivered header when identity alignment is uncertain.
- Model application senders and legacy SMTP before a mailbox-provider cutover.
Questions this hub can answer
- What does this Outlook NDR actually indicate?
- Is the problem sender authentication or tenant policy?
- Which systems depend on the current outbound path?
- Will a provider migration break application email?
Provider documentation
Use the provider’s own documentation for the current provider-defined policy or error meaning, then verify the actual configuration with the tools below.